1. Field of the Invention
The present invention relates to a film thickness measuring apparatus and method for measuring the film thickness of a multilayer thin film.
Priority is claimed on Japanese Patent Application No. 2007-115488, filed on Apr. 25, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
A film thickness measuring apparatus is used for measuring the film thickness distribution and the film thickness error of a multilayer thin film including a flexible substrate, a multilayer composite film sheet, and other kinds of multilayer thin films. FIG. 5 is a diagram showing a schematic configuration of a conventional film thickness measuring apparatus. As shown in FIG. 5, a conventional film thickness measuring apparatus 100 is provided with a white light source device 101, an irradiation fiber 102, a light receiving fiber 103, a spectroscope 104, and a computation section 105. The film thickness measuring apparatus 100 measures the film thickness of a film 200 formed with a plurality of layers 201 to 203.
The white light source device 101 is a light source that emits white light. The irradiation fiber 102 is a light guiding member that guides the light emitted from the white light source device 101 to the film 200 so as to irradiate the light onto the film 200. The light receiving fiber 103 is a light guiding member that guides reflected light from the film 200 to the spectroscope 104.
The spectroscope 104 disperses the reflected light from the film 200 that has been guided by the light receiving fiber 103, and further converts it into electrical signals to obtain reflectance spectrums.
The computation section 105 performs a predetermined computation for the reflectance spectrums obtained in the spectroscope 104 to measure the film thickness of the film 200. Specifically, the computation section 105 performs a computation to select one predetermined wavelength range among the reflectance spectrums obtained in the spectroscope 104, and a computation to obtain wavenumber range reflectance spectrums by re-sequencing the reflectance spectrums within the selected wavelength range at equal intervals. Next, the computation section 105 performs a computation to obtain a power spectrum by performing a Fourier conversion on the signals that indicate the wavenumber range reflectance spectrums. Next, the computation section 105 performs a computation to detect peaks in the power spectrum.
In the above configuration, the white light emitted from the white light source device 101 is guided by the irradiation fiber 102 so as to be irradiated from the layer 201 side onto the film 200. Among the reflected light obtained as a result of irradiating the white light onto the film 200, the reflected light that has been incident on the light receiving fiber 103 is guided to the spectroscope 104 by the light receiving fiber 103. This reflected light is dispersed in the spectroscope 104, and furthermore it is photoelectrically converted. The electrical signal obtained as a result of the photoelectric conversion is inputted to the computation section 105, and is subjected to the various computations mentioned above. As a result of these computations, the film thickness of the film 200 is found.
The white light reflected on each interfacial surface of the respective layers 201 to 203 that form the film 200 has an optical path difference according to the distance between the respective interfacial surfaces (film thickness). Since the white light having optical path differences mutually interferes with each other, a cyclical interference pattern emerges in a wavenumber range reflectance spectrum. A power spectrum that is obtained as a result of performing a Fourier conversion on this cyclical interference pattern has a peak at a frequency according to the optical path difference. Therefore, by detecting this peak, an optical path length difference (optical film thickness) can be found.
FIG. 6 shows an example of a power spectrum obtained by the film thickness measuring device 100. The film thickness measurement result shown in FIG. 6 is a result of measuring the film 200 formed with the layer 201, the layer 202, and the layer 203. The refraction factor of the film 201 is 1.6 and the film thickness of the layer 201 is 5 μm. The refraction factor of the film 202 is 1.7 and the film thickness of the layer 202 is 12 μm. The refraction factor of the film 203 is 1.6 and the film thickness of the layer 203 is 3 μm. The optical film thicknesses of the layers 201 to 203 are found as a product of the film thickness and the refraction factor, and they are 8.0 μm, 20.4 μm, and 4.8 μm. Moreover, in the graph shown in FIG. 6, the horizontal axis represents the optical film thickness, and the vertical axis represents the intensity (arbitrary units).
Referring to FIG. 6, it can be seen that there is a plurality of peaks emerging. The horizontal positions of these peaks indicate the optical distance between the respective interfacial surfaces, and the heights of the peaks indicate products of amplitude reflectances on the two interfacial surfaces. Specifically, in FIG. 6, there are six peaks emerging. The first peak P101 emerges at 4.8 μm indicating the optical film thickness of the layer 203. The second peak P102 emerges at 8.0 μm indicating the optical film thickness of the layer 201. The third peak P103 emerges at 20.4 μm indicating the optical film thickness of the layer 202.
The fourth peak P104 emerges at 25.2 μm indicating the sum of the optical film thickness of the layer 202 and the optical film thickness of the layer 203. The fifth peak P105 emerges at 28.4 μm indicating the sum of the optical film thickness of the layer 201 and the optical film thickness of the layer 202. The sixth peak P106 emerges at 33.2 μm indicating the sum of the optical film thicknesses of the layers 201 to 203 (that is, the optical film thickness of the film 200). Thereby, based on the position of the peak of the power spectrum, the optical film thicknesses of the respective layers 201 to 203 forming the film 200 and the optical film thickness of the film 200 can be found.
For the detail of the conventional apparatus and method for measuring film thickness, refer for example to Japanese Unexamined Patent Application, First Publication No. 2005-308394, and Japanese Unexamined Patent Application, First Publication No. Hei 11-314298.
The positions of the peaks of the power spectrum obtained in the above mentioned conventional apparatus and method for measuring film thickness indicate the optical distances between the respective interfacial surfaces. Therefore, there is a problem that the position of the interfacial surface on which the peak has been obtained with respect to the top most surface (the interfacial surface between the layer 201 and air) cannot be directly found from the power spectrum. That is to say, in the example shown in FIG. 6, the film thicknesses and the refraction factors of the layers 201 to 203 were previously known. Therefore, it was possible to determine that the peak emerging at 4.8 μm indicates the optical film thickness of the layer 203. However, in the case where the film thicknesses and the refraction factors of the layers 201 to 203 are unknown, it cannot be identified that the peak emerging at 4.8 μm indicates the optical film thickness of the layer 203.
In the conventional apparatus and method for measuring film thickness, in the case of measuring a multilayer thin film having a plurality of layers with substantially equal optical film thicknesses, the peaks of the power spectrum overlap with each other. Therefore, the conventional apparatus and method for measuring film thickness has a problem that it is not possible to measure the optical film thickness of only one layer. FIG. 7A and FIG. 7B are graphs showing another example of power spectrums obtained by the conventional film thickness measuring apparatus 100. FIG. 7A and FIG. 7B show results of measuring the optical film thickness of the film 200 formed with the layer 201, the layer 202, and the layer 203. The refraction factor of the film 201 is 1.6 and the film thickness of the layer 201 is 3 μm. The refraction factor of the film 202 is 1.7 and the film thickness of the layer 202 is 12.7 μm. The refraction factor of the film 203 is 1.6 and the film thickness of the layer 203 is 3.2 μm.
Referring to FIG. 7A, reflected light from the interfacial surface related to the layer 201 and reflected light from the interfacial surface related to the layer 203 interfere with each other. This interference generates a peak P201 which is substantially an average of the peak indicating the optical film thickness of the layer 201 and the peak indicating the optical film thickness of the layer 203. Similarly, the interference generates a peak P203 which is substantially an average of: the peak indicating the sum of the optical film thickness of the layer 201 and the optical film thickness of the layer 202; and the peak indicating the sum of the optical film thickness of the layer 202 and the optical film thickness of the layer 203. Referring to FIG. 7B, reflected light from the interfacial surface related to the layer 201 and reflected light from the interfacial surface related to the layer 203 mutually cancel each other. As a result, peaks P301 and P303 with extremely small values are obtained. If a film thickness is measured using such peaks P201, P203, P301, and P303, there will be a problem that a significant measurement error may occur.
In recent years, for high function films and the like, a structure that is symmetric about the center (hereinafter, referred to as symmetric structure) is often used. For example, the following structure is often used. That is to say, a structure “ABA” with the center layer as “B” and with the layer “B” sandwiched by layers “A” of the same film thickness, or a structure “ABCBA” with the center layer as “C”. When the film thickness of a multilayer thin film having such a symmetric structure is to be measured using the conventional apparatus and method for measuring film thickness, there is often a problem in the measurement due to the reason described with reference to FIG. 7A and FIG. 7B. Therefore, there is urgently needed a development of a technique for online-measuring the film thickness of each layer of a symmetrically structured multilayer thin film.
If a manufactured multilayer thin film is extracted and is offline-measured using a vertical scanning type white light interferometer, it is still possible to measure the film thickness of each layer of the multilayer thin film even with a symmetric structure. However, this measuring method has a problem that a measurement at one point requires a period of time ranging from several tens of seconds to several minutes. Moreover, since the measurement takes a long period of time, there is a problem that an error may occur in the measurement result due to vibrations of the multilayer thin film during the measurement, and the error becomes more significant for a thinner multilayer thin film.